In a differential planetary gear apparatus, there has recently been a demand for using a motor, e.g., an inverter motor, as a rotational drive source for speed change in order to accurately control a transmission ratio.
As one of apparatuses which can meet such a demand, there has been known a differential planetary gear apparatus shown in FIG. 14, for example.
The differential planetary gear apparatus shown in FIG. 14 comprises a sun gear Sg fixed to a tip end of an input shaft 55 connected to a drive source 50, a plurality of first planetary gears (pinion gears) P1 disposed radially outwardly of the sun gear Sg and meshing with this sun gear Sg, a plurality of second planetary gears (pinion gears) P2 meshing with the first planetary gears P1, a ring gear Rg having internal teeth which mesh with the second planetary gears P2 being internally contacted with the internal teeth, and an output shaft 65 connected to an edge portion of the ring gear Rg, each of which serves as a basic element of a transmission mechanism.
The differential planetary gear apparatus further comprises a hollow shaft Ca as the basic element. The input shaft 55 passes through the hollow shaft Ca in such a state that the input shaft 55 is rotatable relative to the hollow shaft Ca. The differential planetary gear apparatus further comprises a carrier C. This carrier C comprises an end surface Cb connected perpendicularly to an end of the hollow shaft Ca at the center thereof, and first and second support shafts J1 and J2 extending from the end surface Cb in parallel with the hollow shaft Ca and disposed around the hollow shaft Ca.
The plurality of first planetary gears P1 are rotatably supported by the first support shafts J1, and the plurality of second planetary gears P2 meshing with the first planetary gears P1 are rotatably supported by the second support shafts J2.
A gear Cc is formed on a circumferential edge portion of the end surface Cb of the carrier C, and this gear Cc meshes with braking gears 70 and 80. These braking gears 70 and 80 are connected to brake devices B1 and B3 via rotating shafts 75 and 85, respectively.
The above-mentioned differential planetary gear apparatus shown in FIG. 14 is of a double-pinion type.
However, this double-pinion-type differential planetary gear apparatus has a number of components, and hence the structure thereof becomes complicated and the radial size thereof becomes large. Further, because of mechanical unbalance in the radial direction, this type of differential planetary gear apparatus is unsuitable for a high-speed rotation. Furthermore, since a rotational speed is controlled by the brake devices B1 and B3, its control accuracy is low.
In a case where a large-capacity constant-speed motor is used as the drive source of the above-mentioned differential planetary gear apparatus, in order to start such a constant speed motor, for example, a rotational speed of this drive source is required to be increased to nearly a rated rotational speed (e.g., a rotational speed within ±5% of the rated rotational speed) in some cases. In such cases, another starting drive means is required in order to increase the rotational speed of the drive source (e.g., the large-capacity constant-speed motor) to nearly a rated rotational speed (e.g., a rotational speed within ±5% of the rated rotational speed).
For example, in a case of starting a squirrel-cage induction motor, full voltage (i.e., full-voltage starting) is not preferable because starting current becomes high. Therefore, it is required to provide a reduced-voltage starter utilizing star delta, reactor, Kondorfer, or the like.
Providing such another starting drive means causes an increase in installation cost and other cost, and also causes a complicated structure.
Further, in a case of using a normal constant-speed motor as the starting drive means, if a constant rotational speed of the constant-speed motor, which serves as the starting drive means, is lower than the above-mentioned rotational speed near the rated rotational speed (e.g., the rotational speed within ±5% of the rated rotational speed), some sort of means is required for increasing the rotational speed. Otherwise, it is difficult to increase the rotational speed of the above-mentioned drive source to nearly the rated rotational speed.